1. Field of the Invention
The present invention relates to the field of concentrating dissolved solids by freezing the solvent for the process purpose of mechanical separation. More particularly, the present invention relates to the recovery of fuel and chemical values from wood pulping liquors by freezing the water solvent thereof.
2. Description of the Prior Art
Pursuant of the manufacture of cellulosic fiber products such as paper, the natural lignin structural bond between cellulose fibers is dissolved by a thermo-chemical digestion process.
The aqueous residual from such digestion processes, characterized as black liquor, is laden with the dissolved organic lignin in various degrees of combination with the organic digestion chemicals.
As originally separated from the digested wood pulp, the black liquor solution contains approximately 90% water and 10% solids. These solids are in the form of dissolved organic wood substance and spent digestion chemicals.
Being a carbonaceous material, dissolved wood substance has a heating value of 5000 BTU per pound. However, in combination with the water solvent and other inorganic compounds, this heating value is somewhat reduced. Nevertheless, in a practical black liquor combination comprising approximately 60% solids and 40% water, the dissolved organics contribute a net thermal energy of approximately 2500 BTU per pound.
As a consequence of the heating value contained in black liquor, the substance is used as fuel to support processes whereby the black liquor chemical values are recovered for re-use in the wood digestion step. In the case of low yield pulping wherein a majority of the lignin present in the raw wood furnish is removed, the black liquor energy content is more than adequate to support the entire chemical recovery process by conventional means. Self-supporting recovery plants for low yield pulping, however, are more marginal.
A primary component of conventional recovery plants is a multiple effect evaporator wherein the black liquor solids are evaporatively concentrated from a solution of approximately 10% solids strength to approximately 60% solids strength. The heat required to drive this evaporative concentration step consumes a majority of the available energy in the black liquor.
Due to the fact that the heat of fusion of the black liquor solvent, water, is only 15% of the solvent heat of evaporation, it has been suggested in the past that the black liquor solids concentration step may be conducted more efficiently by means of a freeze concentration process. However, no complete system thereof has been previously devised. Examples of such suggestions may be found in the following U.S. patent disclosures: U.S. Pat. Nos. 1,359,911 to E. Oman; 1,396,028 to W. H. Dickerson; 2,780,281 to A. E. Reinert; 3,098,735 to H. H. Clark; and 3,248,890 to E. Oman.
Relative to techniques and apparatus for concentrating the solid content of other aqueous solution borne substances, reference is directed to the following U.S. patent disclosures: U.S. Pat. Nos. 2,424,663 to H. L. Mantle; 2,617,274 to J. Schmidt; 3,587,859 to R. F. Probstein et al.; 3,824,799 to D. Ganiaris; and 3,835,658 to J. H. Wilson.